To measure the strength of magnetic field, usually a Hall device which utilizes the Hall effect of semiconductor is used.
When a current and a magnetic field are applied to a semiconductor in the directions which meet at right angles with each other, a voltage is induced to a direction which is vertical both to the directions of the current and the magnetic field. The effect is called "Hall effect". The induced voltage is in proportion to the product of the current and the magnetic field. Then, the strength of magnetic field is known from the induced voltage when the current is kept to be a constant, known value. The proportion constant of the induced voltage to the product of the current and the magnetic field is called a "Hall coefficient". In general, the Hall coefficient is in inverse proportion to the carrier concentration in the semiconductor. The induced voltage is often called a Hall voltage. So far, the Hall devices are made from the semiconductors, gallium arsenide (GaAs), indium arsenide (InAs) or indium antimony (InSb). These semiconductors are gifted with high carrier mobilities; 8600 cm.sup.2 /Vs electron for GaAs, 25000 cm.sup.2 /Vs electron for InAs and 76000 cm.sup.2 /Vs electron for InSb at room temperature. The high carrier mobilities can induce high Hall voltages in the Hall measurement. Thus, by employing these semiconductors with high carrier (electron or hole) mobilities to the material of the Hall devices, we can obtain excellent Hall devices with high sensitivity to magnetic field. Therefore, the Hall devices made from GaAs, InAs, or InSb have been widely used in various fields to measure the strength of magnetic field.
It is said that low carrier concentration in the semiconductor is desirable to heighten the sensitivity of the Hall devices to the magnetic field. Usually, the semiconductors with the carrier concentration between 10.sup.15 cm.sup.-3 and 10.sup.16 cm.sup.-3 are used to the material of the Hall devices.
The performance of Hall device is estimated by the "Hall mobility" which is defined as a product of the conductivity and the Hall coefficient. The Hall coefficient is inversely proportional to the carrier concentration as mentioned before. The aforementioned semiconductors, i.e. InSb, InAs or GaAs have been considered as the most preferable materials for the Hall devices, because of their high Hall coefficients and high carrier mobilities. The band gaps at room temperature of GaAs, InAs, and InSb are 1.45 eV, 0.35 eV and 0.18 eV respectively. These semiconductors are quite useful if the Hall devices are used at room temperature. However, if the Hall device would be used at high temperature, these semiconductors would become incompetent by two reasons.
One of the reasons relates to the width of the band gap. The other relates to the thermal problems.
InAs and InSb are totally in the "intrinsic region" at the temperature higher than 500.degree. C., because of the small band gaps. The instrinsic region signifies the region in which the electron concentration and the hole concentration are nearly equal each other. In general, semiconductors are classified into three kinds, e.g. n-type, intrinsic, or p-type semiconductors. The n-type semiconductors have electrons as majority carriers and the p-type semiconductors have holes as majority carriers. The intrinsic semiconductors have both electrons and holes with nearly same concentrations.
The Hall effect occurs only when either electrons or holes exist in the semiconductor. When both electrons and holes coexist in the semiconductor, no Hall voltage is induced because the voltage induced by the electrons is fully cancelled by the voltage induced by the holes. The electron-induced voltage is inverse to the hole-induced voltage, because the voltages derive from the Lorentz force which is in proportion to the product of the electronic charge, the velocity and the magnetic field. Thus, the semiconductor available for the Hall device must be either an n-type or a p-type semiconductor with low carrier concentration.
Therefore, the semiconductors InSb and InAs show no Hall effect above 500.degree. C., because they are intrinsic there. The devices made from InSb or InAs cannot be used as Hall device above 500.degree. C.
The semiconductor GaAs has a wider band gap (1.45 eV) than InAs (0.35 eV) or InSb (0.18 eV). The "intrinsic carrier" concentration in GaAs is nearly 10.sup.15 cm.sup.-3 at 500.degree. C. and nearly 10.sup.16 cm.sup.-3 at 700.degree. C. If the dopant concentration in GaAs would be settled to be higher than 10.sup.17 cm.sup.-3, the Hall devices made from the GaAs would still work at nearly 700.degree. C. despite the deterioration of sensitivity. Here, intrinsic carrier concentration is defined as a square root of the product of electron concentration and hole concentration, which is a common value with an n-type, p-type or intrinsic semiconductor. The intrinsic carrier concentration depends only on the temperature and the band gap. The intrinsic carrier concentration increases according to the rising of temperature. Narrower band gap leads to the higher intrinsic concentration. If the dopant concentration prevails over the intrinsic carrier concentration, the semiconductor can be still either an n-type or a p-type semiconductor.
However, thermal problems further restrict the function of the GaAs Hall devices at high temperature. Above 500.degree. C., the crystalline structure of GaAs is likely to collapse. The chemical and physical properties of GaAs become unstable. Thus, electronic property becomes also unstable. Therefore, the GaAs Hall devices lack stability and endurance at high temperature more than 500.degree. C.
The purpose of this invention is to provide a Hall device which can be used at high temperature.
Another purpose of this invention is to provide a Hall device which works under severe conditions.